Gas tube overvoltage protectors are widely used for the protection of equipment from overvoltage conditions which may be caused by lightning, high voltage line contact, and the like.
It is also a widely practiced technique to associate various fail-safe arrangements with such tubes and with other types of protectors, e.g., air gap arresters, to meet various contingencies. For example, the presence of a sustained overload, as where a power line has come in continued contact with a protected telephone line, produces a concomitant sustained ionization of the gas tube and the resultant passage of heavy currents through the tube. Such currents will in many cases destroy the overvoltage protector and may also constitute a fire hazard.
One common approach to this problem is to employ fusible elements which fuse in the presence of such overloads and provide either a permanent short circuiting of the arrester directly, or function to release another mechanism, e.g., a spring loaded shorting member, which provides the short circuit connection (commonly, the arrester electrodes are both shorted and grounded). The presence of the permanent short and ground condition serves to flag attention to that condition thus signalling the need for its inspection or replacement. Examples of this type of fail-safe protection are found in U.S. Pat. Nos. 3,254,179; 3,281,625; 3,340,431; 3,896,343; and 3,522,570. Several of these patents also incorporate with the fail-safe feature, a backup air gap arrangement so that there is both fail-safe fusible (short) type screw-in or well type arresters are described in U.S. Pat. Nos. 3,703,665 and 3,755,715.
It has formerly been common practice to use gas tubes connected in parallel with conventional, separately mounted, carbon air gaps which break down at less than 1000 V, as back up protection so that in the event that the gas tube leaks and becomes operative at high voltages, usually in excess of 2500 V or more, the air gap will function to maintain the desired protection level.
Assemblies such as those in U.S. Pat. Nos. 3,569,786 and 4,002,952 covered arrangements for providing similar carbon air gaps connected in parallel with the gas tube, but mounted in a unitary screw in protection assembly. These arrangements suffer the well known disadvantages of the carbon air gap which include the development of low insulation resistance and noise conditions on the lines to which they are connected.
The arrangement of U.S. Pat. No. 3,755,715 includes a back-up air gap using metal electrodes in parallel with the gas tube. This arrangement suffers from the disadvantage that it is difficult in manufacture to control the air gap length within the close dimensional limits to ensure that the gap would breakdown at a voltage greater than that of the gas tube and yet breakdown at less than, say, 1000 V if the gas tube leaks.
The present invention seeks to provide an economic assembly which overcomes the disadvantages of the above arrangements and at the same time to provide a seal over the air gap to exclude dust, moisture and other contaminants.